Cod sursă (job #800614)

#include <fstream>
#include <cstring>

using namespace std;

ifstream cin("cristela.in");
ofstream cout("cristela.out");

long long A[(1<<21)];
long long dp[(1<<21)][21];
long long SoS[(1<<21)];

long long getPairs(int n, int m)
{
	long long total = (long long)(n) * (long long)(n - 1);
	int i = 0, j = (1 << m)-1;
	for (; j >= 0; i++, j--)
	{
		total -= (long long)(A[i] * SoS[j]);
	}
	return total / 2;
}

int main()
{
    int n;
    char s[20];
    int sigma=0;
    cin>>n;
    for(int i=0;i<n;i++)
    {
        cin>>s;
        int x=strlen(s);
        int nr=0;
        for(int j=0;j<x;j++)
        {
            if(s[j]-'a'>sigma)
                sigma=s[j]-'a';
            nr=nr|(1<<s[j]-'a');
        }
        A[nr]++;
    }
    unsigned long long cnt=0;
    /*
    for(int i=0;i<(1<<sigma+1);i++)
    {
        if(A[i])
        {
            cnt+=A[i]*(A[i]-1)/2;
            for(int j=0;j<i;j++)
                if(A[j])
                {
                    if(i&j)
                    cnt+=A[i]*A[j];
                }
        }
        
    }
    */
    for(int mask=0;mask<(1<<sigma+1);mask++)
    {
        dp[mask][-1]=A[mask];
        for(int j=0;j<sigma+1;j++)
        {
            if(mask&(1<<j))
                dp[mask][j]=dp[mask][j-1]+dp[mask^(1<<j)][j-1];
            else
                dp[mask][j]=dp[mask][j-1];
        }
        SoS[mask]+=dp[mask][sigma];
    }
    
    cout<<getPairs(n,sigma+1);
    

    return 0;
}